This invention relates to a method and apparatus for analyzing a streak image formed on an electro-optical streaking image tube, and more particularly to a method and apparatus for converting an ultrafast light pulse into a streak image and observing a change in the light pulse intensity with the passage of time.
In laser optics, spectroscopy and other fields where light pulses on the order of several picoseconds or under are analyzed and measured, such ultrafast light pulses are picked up by an image tube streak camera, then their intensity profiles are measured from the streak images produced by the camera. FIG. 1 illustrates a streak camera, in which an incident light pulse L is divided by a beam splitter 1 into two light beams. A beam that advances straight on is narrowed through a horizontal slit 2 (extending perpendicular to the paper).
The image of the slit 1 is projected through a focusing lens 3 on a photocathode 5 of a streak tube 4, and the photocathode 5 emits electrons proportional to the amount of incident light. The electrons are accelerated by an accelerating electrode 6.
Meanwhile, the other beam that is bent perpendicularly by the beam splitter 1 reaches and triggers a ramp generator 8 by way of a mirror 7. The ramp generator 8 generates a ramp voltage synchronous with the straight-advancing light beam. The ramp voltage is applied through a deflector electrode 9 to deflect the electrons emitted from the photocathode 5 in a direction perpendicular to the longitudinal axis of the slit 2. The deflected electrons reach a fluorescent screen 10 and converted to a streak image.
FIG. 2 shows an streak image S on the fluorescent screen 10 viewed in the direction of arrow A in FIG. 1, in which axis T represents time of incidence, axis X longitudinal position of the slit, and brightness intensity of incident light. The streak image S appears only for a very short time that corresponds to the duration of aftergrow on the fluorescent screen immediately after the arrival of the light pulse.
Conventionally, analysis of the streak image has been accomplished by use of an ordinary TV camera having such a camera tube as vidicon. This TV camera picks up the fluorescent screen of the streaking tube along the vertical T-axis and horizontal X-axis by the standard 1-frame, 2-field scanning method. Resulting video signals are directly displayed on a TV monitor for observation and, at the same time, integrated, by scanning line, into light intensity at each time point so as to represent a waveform of the incident pulsed light. Video signals are also integrated only for a limited period of time based on the synchronizing signal in order to obtain a waveform of an incident pulsed light that has passed a specific portion of the slit.
Generally, vertical resolution of the television picture is defined by the number of scanning lines in one frame. In interlaced scanning, the scanning lines of the first and second fields overlap, so that that portion of the storage signals to be read out in the second field which overlaps with the scanning of the first field is erased. Therefore, resolution is defined by the number of scanning lines in one field, too. This drawback is pronounced when picking up such an instantaneous image as streak image. Progressive scanning eliminates this drawback because no signals are read out from the overlapped portion. Consequently, progressive scanning is substantially equivalent to scanning the unoverlapped portion with a fine beam. Accordingly, vertical resolution can be improved by increasing the number of scanning lines and employing the progressive scanning method. But application of progressive scanning not only to the pickup tube but also to the monitor and video recorder forbids the use of readily available or existing standard-scanning-type equipment.